The present invention relates generally to jet engines and more particularly to a support for the turbine of a jet engine which eliminates the need for conventional engine frames and which eliminates the need for shrouds on the high pressure sloped turbine blades.
Jet engines include a core turbojet engine. The parts of a core turbojet engine include a compressor followed in order by a combustor, a turbine, and an exhaust nozzle. The compressor compresses incoming air for better combustion of the fuel injected into the combustor. The burning gases from the combustor turn the high pressure turbine blades to drive the compressor and then exit the exhaust nozzle to supply thrust to the engine.
Certain types of jet engines, such as a bypass turbofan jet engine with the fan at the front (generally called a front-fan engine) have additional engine parts to produce thrust more efficiently. The front-fan engine derives some of its thrust from a core turbojet engine and some of its thrust from an added front fan wherein the outer portion of the airflow from the front fan bypasses the core turbojet engine. Front fans, driven by an added low pressure turbine section (which follows the core engine's high pressure turbine section), act like propellers to supply added thrust to the engine.
The high and low pressure turbine sections each include one or more turbine stages. A turbine stage includes a row of rotating blades whose tips clear the surrounding turbine stator casing. A rotating blade of larger radius produces more power than one of smaller radius. Sloped turbine blades (herein defined to be turbine blades with sloped blade tips) allow the jet engine designer to more easily increase the turbine blade radius in later stages.
Conventional turbine bearing assemblies are of the roller bearing type which experience axial thermal growth in the following manner. A roller bearing assembly has its inner race affixed to a turbine shaft which leads to the turbine blades and has its outer race usually affixed to a turbine frame which leads to the turbine stator casing. One race (such as the outer race) axially captures the roller bearing which is free to axially slide with respect to the other race (such as the inner race) under thermal growth conditions. Thus, as the turbine shaft experiences thermal axial growth, the axial sliding of the roller bearing causes relative axial movement of the turbine blades with respect to the turbine stator casing. Therefore, the sloped turbine blades require blade shrouds attached to their tips to maintain proper tip clearances under axial thermal growth conditions, as is known to those skilled in the art.
The high pressure turbine blades experience higher airflow temperatures and greater tip rotation speeds. A high pressure turbine blade shroud typically puts a centrifugal load of several tons on the blade tip while the blade tip is experiencing high temperature conditions.
In known jet engines, there is often an engine frame whose sole purpose is to support the bearings of the jet engine's turbine. This turbine frame adds weight to the engine. The elimination of the turbine engine frame would result in improved specific fuel consumption and lower manufacturing cost. In other known jet engines without turbine engine frames, turbine nozzle vanes (or support rods running through the vanes) support a turbine bearing of the roller bearing type. Such roller bearing assemblies experience thermal axial growth requiring sloped turbine blades to have blade shrouds attached to their tips to maintain proper tip clearances.
What is needed is a jet engine which supports the turbine bearings without the need for a heavy engine frame and which allows the use of high pressure sloped turbine blades without requiring centrifugal-load-imposing blade shrouds.